Manufacture of asbestos sheet material



Patented Apr. 19, 1949 MANUFACTURE OF ASBESTOS SHEET MAT ERIAL Marion F.Smith and Harold W. Greider, Wyoming, Ohio, assignors to The PhilipCarey Manui'acturing Company, a corporation of Ohio No Drawing.Application January 15, 1945, Serial No. 572,980

1 Claim. 1

This invention relates to-the manufacture of asbestos products andrelates especially to the manufacture of products which comprise fibrousmaterial containing asbestiform mineral fiber disposed in intimatelycontacting relation as by felting or other operation adapted to form asheet or sheet-like body.

The asbestos products which find most extensive commercial use areasbestos sheet materials that are usually produced by water laying andthat are generally referred to as asbestos paper or asbestos mill boardwhich is referred to herein generally as paper. The bulk of theasbestiform mineral fiber that is used in asbestos paper usually runsfrom about to in length, although the fiber that is supplied for papermaking generally contains a considerable quantity of shorter fibers ofvarying lengths and may contain a portion of longer fibers. The mostgenerally accepted system of classification of asbestos fibers is thatof the Quebec Asbestos Producers Association. The asbestos fibers whichare most commonly used for the manufacture of asbestos paper are thosewhich range from the group 5 or paper classification to the group 7 orshorts classification of the Quebec Asbestos Producers Association, ormixtures thereof.

In the manufacture of asbestos paper from asbestos fibers of thecharacter aforesaid, the fibers are water-laid in felted relation by apapermaking operation, a multi-cylinder paper-making machine generallybeing used for the purpose.

An asbestos paper of the character aforesaid has verylittle strength inthe absence of a binder. Hydration by beating, as employed for thedevelopment of strength in cellulosic papers, is ineffective with theinorganic asbestos fiber and merely results in shortening of the fiberwith consequent actual loss of strength of the resulting asbestos paper.The strength of the asbestos paper can be somewhat increased by theemployment of asbestos fibers which are longer than the asbestos fibersusually used in the manufacture of asbestos paper. There are, however,objections to the use of long asbestos fibers in the manufacture ofasbestos paper. In the first place, long asbestos fibers are of muchhigher cost and are usually reserved for spinning purposes. Asbestosfibers of intermediate length,

namely, between th long spinning fibers and the relatively short fibersused for making asbestos paper, are generally used for reinforcementpurposes, e. g., as a reinforcement in the manufacture of heatinsulation materials and the like which consist in major proportion offinelyincrease in strength is afforded by the employment of longasbestos fibers in the manufacture of asbestos paper. This invention isof particular utility in the manufacture of products from the morecommon and less expensive fibers of the paper-making grades mentionedabove.

It has heretofore been standard practice in the manufacture of asbestospaper to use starch as the binder material for imparting strength to thepaper, since starch has been found to be the most economical andeffective binder for webs comprising asbestos fibers. The starch may beused in varying amounts depending upon the strength to be imparted tothe asbestos paper andv depending upon the purpose for which theasbestos paper is intended. In addition to the starch, other organicbinder materials have had limited use in connection with asbestos paper,such as rubber latex and various synthetic rubber-like materials whichare generally referred to as elastomers. Ordinary organic binders suchas casein, soya protein, glue, rosin and the like have not been found tobe suitable for use in the manufacture of asbestos paper. Syntheticresins of various types may be used as a binder for asbestos fibers buttheir cost is extremely high and for this reason their use for mostpurposes is not practical.

The use in asbestos paper or other asbestoscontaining products of anorganic binder material such as starch is objectionable for severalreasons. The most serious objection is due to the fact that an organicbinder material lacks the high resistance to heat that is possessed byasbestiform mineral fibers and that constitutes the main reason for useof such fibers instead of the much less expensive organic fibers.Asbestos fiber is capable of withstanding sustained temperatures ofabout 900 F. to 1000 F. without excessive deterioration. Moreover,asbestos fiber, being inorganic, is non-combustible, and can be exposeddirectly to a flame without buming orsmoking. Other advantages ofasbestos fiber which make its use desirable result from the fact thatasbestos fiber is unaffected by water and is not subject to rotting orother gradual Moreover, ashes deterioration due to moisture. tos fiberis not subject to attack by organisms which result in mildew, mold,fungus growths or the like and is not attractive to, or destroyed byvermin.

When an organic binder is usedin conjunction with asbestos fiber, itspresence as a binder is disadvantageous since an organic binder lacksvirtually all of the desirable properties of the asbestos fiber. Anorganic binder, if present in an asbestos paper, will start to decomposegradually at temperatures as low as 250 F. during period of prolongedexposure. Temperatures of 300 F, are actively destructive to organicbinder materials contained in asbestos paper, while at temperatures of350 F. an organic binder will char in a matter of a few hours time. Attemperatures of 400 F. and higher an organic binder will smoke and charimmediately. when the organic binder in an asbestos paper or otherproduct is disintegrated due to temperature conditions such as thoseabove mentioned, it loses its efiectiveness as a binder with the resultthat the asbestos product becomes lacking in strength to an excessivedegree and goes to pieces. As a result of this fact, the use of asbestospaper containing starch or other organic binder material for heatinsulation purposes has been limited to conditions such that onlyrelatively mild temperatures are encountered, namely, temperatures underabout 300 F. As far as the asbestos fiber itself is concerned, it couldbe used at considerably higher temperatures, namely, temperatures up toabout 900 F. to 1000 F. and would have wide commercial utilityfor use atsuch temperatures, but no binder has heretofor been found which issuitable as a binder for asbestos paper and which'will not disintegrateexcessively at temperature above about 300 F.

The extent to which an organic binder is ob- Jectionable because of itslack of resistance to heat depends in part upon the amount of binderthat is used. If the amount of binder is relatively low, such as of theorder of about or less by weight of the asbestos paper, the binder willnot burn so as to produce a flame but it will discolor and also willchar and smoke in a manner that is objectionable. If additional organicbinder is used, e. g. to by weight, the binder in the asbestos paper mayactually burn with a free flame. Regardless of the amount of binder thatis used, any organic binder, such as starch, rubber, glue, resin or thelike tends to give off suffocating fumes when heated, such fumes being ahazard to life and objected to by Underwriters Laboratories, Inc., andby the National Board of Fire Underwriters as dangerous to publicsafety. Moreover, some gases which are given off are combustible and mayeven be explosive and present a further hazard for this reason. The moregradual deterioration and disintegration of organic binder material, asa result of prolonged exposures to elevated temperatures will, ofcourse, take place regardless of amount of binder that is employed.Other objections to the use of organic binders are lack of resistance towater and moisture, rotting, molding, and attractiveness to vermin.

Summarizing the foregoing, the use in paper or other products ofasbestiform mineral fibers is highly advantageous due to the resistanceof asbestiform mineral fibers to heat, to water, to

rotting and other deteriorating influences. How-' ever, when organicbinders are employed, the organic binders are lacking in thecharacteristic properties which make the use of asbestos fibersdesirable and the uses to which the asbestos product can be put becomelimited due to the undesirable properties and limitations of the bindermaterial used.

It is possible to use certain inorganic binders with asbestos fibers butthe products which result from the use of inorganic bindermaterials havebeen limited to rigid and boardy products which are cementitious incharacter.

It has also been proposed to impregnate asbestos paper with solublesilicates, such as commercial sodium silicate solution, and then dry theresultant sheet, the silicate, upon drying, becoming of a hardenedglass-like character so that the dried product is rigid, boardy andbrittle, and cracks readily when flexed.

It is the principal purpose of this invention to bond together thefibers of an asbestos product without the use of an organic binder andwithout rendering the product stiff and boardy as is the case whensubstances such as Portland cement or soluble silicates are used asbinder materials. It is a further purpose of this invention to affordasbestos products such as asbestos paper, sheets or the like having highresistance to heat, moisture, rot and other destructive influences. Itis a further object of this invention to provide novel heat-insulationmaterial, board-like products, and other products fabricated from sheetsof felted asbestos fibers that have been interbonded according to thisinvention.

We have discovered that asbestiform mineral fibers which have beenproduced in the form of' a felted sheet-like body can be bonded togetherby the interaction of they asbestiform mineral fiber with a solution ofa water-soluble inorganic fluoride to form a coherent body that isflexible and porous but that has much more strength 40 than an untreatedsheet of felted asbestiform mineral fibers. The interaction that occursbetween the solution of the fluoride and the asbestiform mineral fiberis not definitely understood, but appears to be specific between thesubstance of the asbestiform mineral fibers and the fluoride. Numerousother inorganic compounds do not have such effect and may even decreaserather than increase the strength of the felted sheet material. Whilethere appears to be some action'of the fluoride on the surface of theasbestiform mineral fiber which affords a bonding material that bondsthe asbestiform mineral fibers together where they come into contactwhen the fibers are subsequently dried en masse,

. the asbestiform mineral fibers retain their characteristic fibrouscharacter and ,the resulting felted sheet after treatment has thedesired bibuions and flexible character of asbestos paper.

The manufacture of a strong and coherent asbestos paper without theemployment of any organic bindr according to this invention may beillustrated as follows. Asbestos fiber, which may be any of the usualpaper grades of asbestos fiber heretofore used in the manufacture ofasbestos paper products, is made up into an aqueous furnish according toconventional methods used in the manufacture of asbestos paper and thefurnish is formed into sheet material on a papermaking machine in theusual way until an asbestos paper is produced having the ultimatethickness and weight .desired. The paper thus produced is free of anybinder and after it has been formed it is subjected to drying as bypassing it over a plurality of drying rolls. According to thisinvention, the asbestos paper, which I or the like. The extent to whichthe paper is dried before the acid solution is applied may be merelysuiflcient to enable the acid to penetrate into the paper. Preferably,however, the paper is substantially completely dried (so that it willcontain less than about by weight of moisture) before the acid isapplied, since by so doing the absorption of the acid into the paper ismore complete and is more uniform; After the acid solution has beenapplied, the paper is again dried as by passing it over the drying rollswhich may be heated to conventional drying temperatures such as 200 to300 F. although the degree of heating is in no way material anddoes notcontribute to the successful manufacture of the new product. If desired,the product may be dried at ordinary atmospheric temperature. The driedpaper may, for example, contain about 5% or less of retained moisturealthough the extent of drying is not critical. After the paper has beendried, it is ordinarily wound on-a reel, trimmed to desired width, andmade up into rolls as is conventional in paper-making operations.

The above-described process can advantageously be carried out in asingle and continuous operation by applying the hydrofluoric acid solu-.

tion to the asbestos paper at an intermediate.

stage during the passage of the paper over the drying rolls of aconventional machine for the manufacture of asbestos paper products.

The concentration of the acid solution that is applied to the asbestospaper is not critical. Usually, the hydrofluoric acid as applied isdiluted with water so as to be of about to about 30% concentration. Thestrength of the asbestos paper product is increased somewhat uponincreasing the concentration of the solution that is applied thereto upto a concentration of about 30%, but the amount of strength imparted ininterbonding the asbestos fibers is not proportional to theconcentration of the solution em ployed and the concentration of thesolution employed does not appear to be critical.

The effectiveness of the bonding that may be afl'orded between thefibers of an asbestos paper may be illustrated in connection with thefollowing example. If asbestos paper is made by a conventionalpaper-making operation so as to weigh about ten pounds per- 100 squarefeet, the resulting web or sheet when dried, and without having had anybinder included in the furnish has a tensile strength of only about twopounds per linear inch of width in the machine direction of the-sheetand a tensile strength of only about one-half pound per linear inchacross the sheet. Upon applying hydrofluoric acid solution of aboutconcentration to the sheet material so that the sheet material will takeup about seventy pounds of the hydrofluoric acid solution for each onehundred pounds of the asbestos sheet and then drying the sheet, theresulting product has a tensile strength in the machine direction of thesheet of about 11 pounds per linear inch of width and about 4 pounds perlinear inch across the sheet. Thetensile strengths that are given aboveand elsewhere herein are as determined by the method prescribed in A. S.T. M. standard D-202-41T, the test specimens of paper having beenconditioned 8 at 45% relative humidity and 77 F. for four hours beforetesting.

Asbestos-paper or other felted sheet-like body wherein the asbestiformmineral fibers have been bonded together by the interaction in situbetween the asbestiform mineral fibers and the hydrofluoric acidsolution, has the advantage of retaining its structural integrity and a.substantial part of its dry strength when contacted with water. A verysevere accelerated test for deter-rv mining the resistance of asbestospaper to water consists in immersing a small sample of the product(about 1 x 2 inches) in boiling water. Failure, if it occurs, is takenas the point at which the binder no longer acts to hold the fiberstogether, the fibers becoming liberated to form a pulp mass. whensubjected to the boiling water test, the paper, wherein the fibers hadbeen bonded together by the action of hydrofluoric acid solution,successfully withstood the boiling water test for over 90 minutes. Thetest was discontinued at that time because the paper had notdisintegrated and there was no indication that longer exposure toboilingwater would result in disintegration. The high resistance towater that is afiorded by the bonding together of the asbestiformmineral fiber by the action of hydrofluoric acid solution thereon insitu is of great practical advantage in that asbestos paper having highwet strength can be readily produced. By comparison, ordinary asbestospaper which has been bonded by means of starch has virtually noresistance to water and even when moistened with cold water immediatelyloses its strength and becomes reduced to a plastic pulplike mass. Whilea water-insoluble binder such as rubber may be used in asbestos paper inorder to afford improved water-resistance, the higher cost of rubber isapractical disadvantage and, of

course, rubber, being organic, tends to impair fire resistance and tendsto give all malodorous smoke when heated.

In addition to hydrofluoric acidyother watersoluble inorganic fluoridesare effective in the practice of this invention to afford a bondedasbestos sheet or the like. Examples of such compounds which, likehydrofluoric acid, are particularly effective are the fluorides ofammonium and of potassium, namely, ammonium fluoride, ammoniumbifiuoride, potassium fluoride, and potassium bifluoride. Other examplesof water soluble inorganic fluorides are sodium bifiuoride and sodiumfluoride. The substances above listed are salts of hydrofluoric acidwhich afford desired tensile strength, but are less effective thanhydrofluoric acid in providing bonded asbestos sheets and the likehaving high wet strength.

The foregoing substances are illustrative of those compounds which arereferred to herein and in the claims as water-soluble inorganicfluorides. This class of materials is confined to hydrofluoric acid andthe water soluble inorganic salts thereof, such as exemplified above,and does not comprise complex fluorine compounds such ashydrofluosilicic acid or salts thereof.

The water -soluble inorganic fluorides of the type above mentioneddiffer somewhat in efiectiveness and for this reason the concentrationof the solution that is appropriate for the different compounds in theclass varies but, generally speaking, is of the order that has beenmentioned in connection with the employment of hydrofluoric acid. Awater-soluble inorganic fluoride, as this term is used herein and in theclaims, is to be understood as being capable of producing an aqueoussolution of at least about concentration.

A sirably constitute less than 20% by weight of the asbestiform mineralfiber. For the preparation 7 of flexible bibulous asbestos paper it isordinarily desirable that the paper contain less than by weight ofmaterial other than the fiber and any finely-divided filler that may becontained therein.

According to this invention, asbestos paper can be readily manufacturedwhich has ample strength for the purposes to which asbestos papers aregenerally intended. Such paper can be readily handled and subjected tovarious treatments and will withstand deformation as in the manufactureof thermal insulation sheet materials comprising one or more layers ofasbestos paper which has been corrugated or indented or has otherwisebeen deformed. When asbestos paper is prepared without anybinder, itstensile strength in the machine direction is quite low, usually of theorder of one to two pounds per linear inch of width. An increase instrength which doubles the strength of the paper without any binderconstitutes a considerable practical improvement although a tensilestrength of at least five pounds per linear inch is ordinarilydesirable. As pointed out above, considerably higher strengths canreadily be attained according to this invention.

Asbestos sheet material can be made according to this invention which isnotable not only for its strength but also for the fact that it remainsflexible and bibulous. Thus, asbestos sheet material can be madeaccording to this invention substantial strength and which has the heatrewhich takes up water or other liquid material as rapidly as asbestospaper which has been'bonded with, for example, five to ten per cent ofstarch. Typical embodiments of this invention will take up 30% or moreof water and preferably 40% or more of water when immersed in water at77 F. for five minutes. The flexibility of paper and other sheetmaterials made according to this invention is also important in enablingthe paper to be fabricated into articles. In referring to sheet materialas being flexible, it may be mentioned as typical that sheets having athickness up to .050 inch or less may be bent 180 around a mandrel of1.5 inches diameter in two seconds at 77 F. without rupture or breakingat the surface and thus are of a suitable degree of flexibility forfabrication purposes. Thin sheets are, of course, more flexible thanthick ones and the flexibility of the asbestos sheet material madeaccording to this invention can, if desired, be further increased bycalendaring or other manipulative treatment of the sheet after it hasbeen dried. When the asbestos paper is treated with an acidic compoundof the class above defined,- such as hydrofluoric acid or .an acid saltthereof, such treatment to which the asbestos paper is subjectedaccording to this invention does not result in objectionable acidity ofthe product. Asbestos fibers are normally slightly alkaline and thealkalinity of the asbestos fibers eventually neutralizes any aciditywhich is imparted to the paper by the applied acid oracid salt so thatthe ultimate product is either neutral or of the slight alkalinity whichis characteristic of any asbestos sheet consisting sistance and otherproperties of a product made essentially of asbestos. In other words, anasbestos product can be produced according to this invention which isessentially free of organic binder or other organic material and whichis free from the objections that are incident to the presence of organicmaterial in the product. The material may be subjected to sustainedtemperatures of 900 to 1000 1''. without deterioration. Moreover, atsuch temperatures, or even higher temperatures such as flametemperatures, there is no charring, discoloration, or production ofobjectionable smoke or fumes. Moreover, the material is resistant towater and to moisture and is not subject to attack by organisms orvermin.

One of the uses to which the product of this invention is particularlyadapted is use as or in heat insulation materials of various kinds. Thus'the new asbestos paper by itself may be used as a protective coveringfor pipes, furnace walls and the like and may be used even thoughtemperatures as high as 900' to 1000 F. may be encountered. The newasbestos paper may also be used in the fabrication of compositeinsulation products. In such products, the new asbestos paper may, forexample, be used as a surface layer. The new paper of this invention mayalso be utilized as the body portion of the heat-insulation material.

In addition to the above, the new asbestos paper and sheet material ofthis invention may likewise be used in the manufacture of board-likematerials other than those especially designed for heat-insulationpurposes.

It is also apparent that the new product of this invention, when madewithout employment of organic material contained therein, is superior toordinary asbestos felt used in the manufacture of roofings such, forexample, as built-up roofings which are commonly prepared using aplurality of plies of asbestos paper saturated with asphalt and bondedtogether with a suitable mopping asphalt. Since the asbestos paper maybe made according to this invention which is without any combustiblematerial contained therein and which remains coherent notwithstandingthe fact that the asbestos paper is subjected to very high temperature,it is obvious that improved fire resistance can be afforded by using asa base for roofing the new asbestos sheet material of this inventioneither dry or impregnated with a waterproofing material such as abituminous material.

It is not necessary that the new product of this invention be fabricatedin the manner above described, namely, by the water-laying of a feltedsheet of asbestos fibers and the application of the treating compoundthereto. Thus, the asbestos fibers may be brought intointimately-associated felted relationship in other ways either wet ordry. In this connection, operations such as carding, garnetting, and thelike which accomplish a i deposition of fibers to form a sheet-likebody, are

to be regarded as providing felted fibers as the term felted is usedherein and in the claims. The moment of application of the water-solubleinorganic compound for reaction with the fibers is not'important so longas the fibers as disposed in felted relation are in contact with thetreating substances as contained in an aqueous solution and the feltedfibers are permitted to dry en masse so that the bonding materialoccurring at the surfaces of the asbestiform mineral fiber will hardenand serve to interbond the asbestiform mineral be used, namely,chrysotile asbestos fiber.

fibers at the points of contact between the fibers.

paper or the like to which the treating compound has been applied andwhich is still in a wet state may be plied together so as to form aproduct consisting of a plurality of plies, the plies being bondedtogether when dried by the product ofinteraction between the asbestiformmineral fibers and the treating compound.

In the ordinary case, according to this invention, the usual asbestosfiber of commerce may According to this invention and as used in theclaims, the term asbestos is intended to include, in addition tochrysotile asbestos, other commercial varieties .of asbestos, namely,anthophyllite,

actinolite, tremolite, crocidolite, amosite, various amphibole fibersand Canadian picrolite.

While it is a principal advantage of this invention that an asbestosproduct can be produced which is essentially free of organic materialwhich is used either as binder or as part of the fibrous content of theproduct, the advantages of this invention may be availed oi even thoughsome organic material may be present in the product.

For example, an asbestos paper containing a minor amount of organicfiber such, as ordinary cellulosic paper fiber may be subjected-to thetreating step according to this invention and such treatment will resultin the interbonding of the asbestos fibers in the sheet. However, formost purposes where high fire resistance is desired, less than 5% byweight of the fiber should be organic fiber. It is possible to includein the asbestos paper other mineral fibers such as rock wool, slag wool,glass fibers and the like which are heatresistant, but such fibers havethe disadvantages of being more brittle and frangible than asbestosfibers. However, in any event, the content of asbestos fiber should besufficient so that the asbestos fibers in the fibrous product come intointimate association, thereby permitting the asbestos fibers to becomebonded together at a multiplicity of points of contact between theasbestos fibers for the creation of a bond between the asbestos fiberswhich imparts coherency and strength to the product as a whole. As ageneral rule, the product produced according to this invention shouldconsist in major proportion by weight of asbestiform mineral fibers.

The new fibrous product of this invention as ordinarily made up for themarket does not contain any organic binder. However, an organic bindermay be applied depending upon the in tended use of the product. anadvantage of this invention that the usual .binder that is used in themanufacture of asbestos paper, namely, starch, may be omittedaltogether, it is not without the scope of this invention to employ thespecial treating compounds in conjunction with starch. However, even insuch case, it is normally desirable to take advantage of this inventionby reducing the quantity of starch below that which is ordinarily used.Thus, for example, the asbestos paper may contain 1% or less by weightof starch without detracting m'aterially from the heat resistance of thepaper for in such case the paper, when exposed to high temperature, maydiscolor to some extent but will not give off fiame or an undue amountof smoke, and the paper will retain its bond notwithstanding thecarbonization of the starch. These comments are equally applicable toother organic binders. It is ordinarily desirable that the asbestospaper or the like be made up em- Thus, while it is ploying less than 2%of organic binder. As mentioned above, an asbestos paper according tothis invention may, for example, be impregnated with a bituminoussaturant for various purposes. Other types of binder, which may or maynot be organic, may be applied to the sheet material as in themanufacture of gasket material, brake linings and the like. Moreover,the product of this invention may be impregnated with other types ofbonding material such as soluble silicates which become hardened whendried and which may, if desired, be insolubilized by such expedients asthe useof suitable insolubilizing agents or by heat curing.

In addition to the fiber and bonding components of the products producedaccording to this invention, the product may include a minor quantity ofa finely-divided filler material. For example, a small quantity, e. g.,of the order of 5% to 10% of the weight of the fiber, of a material suchas diatomaceous earth may be employed. A filler such as diatomaceousearth does not have an adverse effect on the porosity and absorptivenessof the paper and usually increases these properties. Another fillerwhich affords considerably porosity is fine pumice. Moreover, otherfillers such as clay, talc, pigments to impart suitable color, etc. maybe employed. When the product of this invention is designed to beresistant to high temperatures, the filler material should be heatresistant, namely, should not decompose and char when exposed totemperatures of the order of 900 F. Ordinarily, if the filler materialis heat resistant, an inorganic filler is employed. Ordinarily, themajor proportion by weight of the fiber plus the filler contained in theproduct should consist of asbestiform mineral fibers and,

vfor providing resistance to heat, the fiber plus any filler shouldcontain less than 10% of organic material or other non-heat resistantmaterial.

Since the product of the invention can be made up so as to consistsubstantially of asbestos fiber, or asbestos fiber together with otherheat resistant fiber or filler, the product can be exposed totemperatures such as 900' F. to 1000 F. without injury. This is ofconsiderable advantage in enabling asbestos paper products which havebeen impregnated with a soluble silicate to be heat cured attemperatures of the order mentioned to insolubilize the silicate. Ifordinary asbestos paper containing an organic binder were to besubjected to such heat curing temperatures, the paper would becomediscolored, charred and weakened and an unsatisfactory product wouldresult.

For most purposes where resistance to exposure to high temperatures isdesired, it is desirable to produce the product of this invention so asto be substantially free of any organic material. This is also desirablein order to afi'ord high resistance to rotting, mildew, etc. However,about 6% by Weight of organic material can be tolerated in the productwhile still aifording these attributes in an unusually high degree.

While this invention has been described in connection with certaintypical examples of the practice thereof, it is to be understood thatthis has been done merely for purposes of illustration and that thescope of this invention is to be defined by the language of thefollowing claim.

We claim:

In the manufacture .of a coherent sheet-like body of felted fibersconsisting in major proportion by weight of asbestos fibers, the stepscomprising contacting the asbestos fibers while l1 disposed in saidsheet-like body of felted fibers with an aqueous solution of awater-soluble inorganic fiuoride comprising hydrofluoric acid, saidasbestos fibers b01118 initially contacted with said aqueous solution ofa water-soluble inorganic 5 fluoride after the fibers in saidfelted-fiber sheet like body have been disposed in felted relation insaid sheet-like body, and thereafter drying the sheet-like body of,felted fibers, thereby bonding together the contactinz" asbestos fibersin said sheet-likebod'y essentially by interaction in situ between saidasbestos fibers as disposed in felted relation in said sheet-like bodyand said'water- "soluble inorzanic fluoride comprising hydrofluoricacid.

MARION 1''. SMITH.

HAROLD W. GREIDER.

12 REFERENCES CITED The following references are of record in the fileof this patent:.

UNITED STATES PATENTS 7

